1. Field
This disclosure relates to radio frequency filters using surface acoustic wave (SAW) resonators, and specifically to transmit filters and duplexers for use in communications equipment.
2. Description of the Related Art
As shown in FIG. 1, a SAW resonator 100 may be formed by thin film conductor patterns formed on a surface of a substrate 105 made of a piezoelectric material such as quartz, lithium niobate, lithium tantalate, or lanthanum gallium silicate. The substrate 105 may be a single-crystal slab of the piezoelectric material, or may be a composite substrate including a thin single-crystal wafer of the piezoelectric material bonded to another material such as silicon, sapphire, or quartz. A composite substrate may be used to provide a thermal expansion coefficient different from the thermal expansion coefficient of the single-crystal piezoelectric material alone. A first inter-digital transducer (IDT) 110 may include a plurality of parallel conductors. A radio frequency or microwave signal applied to the first IDT 110 via an input terminal IN may generate an acoustic wave on the surface of the substrate 105. As shown in FIG. 1, the surface acoustic wave will propagate in the left-right direction. A second IDT 120 may convert the acoustic wave back into a radio frequency or microwave signal at an output terminal OUT. The conductors of the second IDT 120 may be interleaved with the conductors of the first IDT 110 as shown. In other SAW resonator configurations (not shown), the conductors forming the second IDT may be disposed on the surface of the substrate 105 adjacent to, or separated from, the conductors forming the first IDT.
The electrical coupling between the first IDT 110 and the second IDT 120 is highly frequency-dependent. The electrical coupling between the first IDT 110 and the second IDT 120 typically exhibits both a resonance (where the impedance between the first and second IDTs is minimum) and an anti-resonance (where the impedance between the first and second IDTs is maximum). The frequencies of the resonance and the anti-resonance are determined primarily by the pitch and orientation of the interdigitated conductors, the choice of substrate material, and the crystallographic orientation of the substrate material. Grating reflectors 130, 132 may be disposed on the substrate to confine most of the energy of the acoustic waves to the area of the substrate occupied by the first and second IDTs 110, 120.
SAW resonators are used in a variety of radio frequency filters including band reject filters, band pass filters, and duplexers. A duplexer is a radio frequency filter device that allows simultaneous transmission in a first frequency band and reception in a second frequency band (different from the first frequency band) using a common antenna. Duplexers are commonly found in radio communications equipment including cellular telephones.
FIG. 2 is a block diagram of portions of a communications device 200. The communications device 200 includes a transmitter 210, a duplexer 220, and antenna 230, and a receiver 240. The duplexer 220 may include a transmit filter 222 and a receive filter 224. The transmit filter 222 may be coupled between the transmitter 210 and the antenna 230. The receive filter 224 may be coupled between the antenna 230 and a receiver 240. An important function of the duplexer 220 is to isolate the receiver from the transmitter to ensure the receiver is not overloaded by energy from the transmitter. To this end, the transmit filter 222 may be designed to pass frequencies in a transmit frequency band and block, or reject, frequencies in a receive frequency band separate for the transmit frequency band. Conversely, the receive filter may be designed to pass frequencies in the receive frequency band and block frequencies in the transmit frequency band.
The transmitter 210 may include a power amplifier 212 that generates the radio frequency signal to be transmitted and an impedance matching network 214 to couple the radio frequency signal from the power amplifier 212 and the transmit filter 222 within the duplexer 220. The impedance matching network 214 may be design to match the output impedance of the power amplifier 212 to the input impedance of the transmit filter 222. Although shown as a portion of the transmitter 210, the impedance matching network 214 may incorporated, in whole or in part, in the transmit filter 222. The output impedance of the power amplifier 212 is typically constant, or nearly constant, over the transmit frequency band. To ensure efficient coupling of power from the power amplifier 212 to the antenna 230, it may be preferable for the input impedance of the transmit filter 222 to also be constant, to the extent possible, over the transmit frequency band.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is first shown and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.